Subfloor thickness

I am having to replace my top subfloor because it was too much work to prep it for tile as the three layers of linoleum that the previous owners had left was just too much to handle. Anyway when I got to the first layer of subfloor it is 1/2". What would you recommend putting on top of that to make it nice an sturdy for tile? What would the professionals do to get it ready. I am going to be using 12x24" tiles with electric radiant heat below. Thanks.

Having 1/2" as the first layer makes things problematic. But, first, you need to determine if the joists are strong enough for tile. Note, there's a difference between ceramic and natural stone in those requirements...few homes are stiff enough for stone out of the box unless designed for it from the beginning. So, to determine the joist stiffness, you need to know the length of the unsupported run, the joist spacing, the height and thickness, and the type of wood. There are calculators that can help you figure that out, or span tables. One I like is at www.johnbridge.com , enter the forums, and in the blue menu bar, click on 'Deflecto', then enter your numbers to check the suitability of the joists for tiling. Note, the length is NOT the size of the room, it's the whole distance between supports UNDERNEATH the floor.

1/2" is too thin to support tile in between the joists ( the joists must be strong enough along their length as well). But, screwing a second layer to 1/2" is tough, as the screws often tear out, as there's not much 'meat' to anchor in. One way to handle this is to put a second layer that is thick enough for tile on it's own, and just ignor the first layer is there. A second way is to cut out the existing stuff, and put new stuff down on the joists. A third way is to carefully pre-drill the top sheet so you can feel better when you've reached a tight anchor with the bottom sheet before you tear out the threads. If the 1/2" stuff is full of holes or has a 'D' face, it shouldn't be counted on as good for tiling - a 'D' face can have internal voids as well, and shouldn't be used under tile.

once you've got the subfloor set, then you need either a cbu, or a decoupling membrane. With the radiant, I like a membrane better as it will account for the greater expansion and contraction caused by the heat source better than cbu. Personally, I like Ditra from www.schluter.com, but there are others that work well. On an older house, it is not uncommon for the floors to be uneven and not level. Tiling a large format tile like you are considering really begs for a super flat floor, otherwise, lippage will be hard to control, especially for a DIY'er. For that, I'd suggest embedding the heating mats/wires in SLC. This will flatten and level the floor in one step. Then the decoupling layer, then the tile.

So, would a 3/4" on top of the 1/2 be a good thickness. Then put down the radiant and the ditra on top of that? I will be using porcelain tile. The calculator tells me tile is going to work for my floor structure so I should be fine. How would you do it? I am not going to take out the 1/2" though. Not an option.

3/4" by itself is fine for ceramic tile. The thinnest you could use is 5/8" to ignore the first layer. The hard part with the heating is getting it flat after you've fastened it to the floor. You need to cover it with something, slc, or thinset. While you can spread thinset and then install the Ditra in one step, it's easier if you embed the heating in the thinset, let it cure, then install the Ditra and tile.

Normally, when trying to make the subfloor stronger and to provide decoupling to isolate the tile, you offset the second sheet and just anchor it to the sheet below, avoiding the joists. But, in your case, since you don't want to remove the 1/2", I'd ignore that it is there, and fasten the second sheet to the joists as if it was the first layer. When you choose one thick enough to support tile on its own, you should be fine. It would not work if you wanted to put down stone tile, but with ceramic, it will be fine. You don't want to mix the thinset thin - sounds like you are considering essentially pouring it on...won't work well. I'd either embed it in slc, which is designed for that, or use thinset, using the wires as a screed (would work if you used a board, you'd risk nicking the insulation with a standard metal trowel). Let that cure, then install the ditra, and then the tile. If you didn't let the thinset cure over the wires first, the thickness would be pretty deep, and you'd likely get divots when kneeling on it trying to embed the fleece of the Ditra into the thinset. Much easier if it is already flat and hard. Thinset adheres to thinset fine. I'd rather use slc, but over a wooden subfloor, it (depends on the brand) needs to be 1/2" over the top of the highest point, or in your case, 5/8", which would raise the floor probably more than you wanted, but would be nice and flat. that assumes the floor is level, it could be more if it isn't.

First, check on the tile forum. Then check with the supplier of the tile.

The rule of thumb that has often been quoted to me is that you need an inch and a quarter. 3/4 over 1/2 would give you that, but it has been mentioned that you might have a problem with securing the top layer ( the situation more commonly seen is putting 1/2 over 3/4, and that can be screwed down well )

INdustry standards allow as little as 5/8" of good quality ply, installed properly under ceramic tile. More is always better. When it comes to a natural stone tile, you need two layers, the first must be at least 5/8" and the second at least 3/8", offset from each other, with the top layer NOT anchored to the joists. Since you are going with ceramic tile, the two layer requirement doesn't apply, but will make the floor stiffer....just not required for ceramic tile. Since it is so hard to get a good attachment to ply that is only 1/2" thick, for your purposes, you could ignore it is even there, and install an adequate subfloor on top of it, anchoring through to the joists. Extra anchors in the field wouldn't hurt, but may be difficult to achieve because of the thin material underneath.

If you wish to minimize the buildup in height, you could remove the existing 1/2" layer, and install 3/4" T&G material, with the edges supported by new blocking where required, if you can't end them on a joist (common problem when you come up against a wall). that would be adequate for ceramic tile. This would give you an opportunity to maybe sister, plane, or shim the existing joists to give you a perfectly level and flat new subfloor. To improve the overall strength, use construction adhesive on the joists. As noted earlier, the bigger the tile, the more important it is to have the floor flat to minimize lippage of those large tile. Large tile are not the easiest thing to lay until you have some experience. You may also want to invest in one of the tile leveling systems such as Tuscan Leveling System, or LASH (by QEP - available at HD). this would make it easier to get the edges perfectly level. Also note, large format tile are sometimes cupped or warped, so perfectly level is impossible. This is a function of the quality of the tile and mostly resulting from inconsistent handling prior to and the operation of firing in the kiln.

If you want more exposure to tile experts, go to www.johnbridge.com, but also being a member there, I think I've got this right.

I won't be laying the tile is what I have decided. I will be putting in the subfloor,(3/4 and 1/2 on top of that), the radiant wire, and then the ditra. I will let the tile man do his thing for that. Does this sound acceptable. Would running 2x 8 between the joist to connect one to the other would that be considered sistering or what? And would that help for more support?

Sistering means adding a second joist along-side an existing one. The goal is to strengthen the joist. It does not have to extend the full length, but needs to cover at least the middle 2/3'rds of the unsupported length to provide the needed support. Blocking goes between joists at 90-degree angles. You may need this if you cut out the existing subflooring to then support the ends or edges if they can't be supported by existing joists or the T&G of the new panel being installed. Blocking does not appreciably increase the floor deflection ratings. It does prevent the joists from twisting, but that often isn't an issue if there's sheet material on both sides and you've used adhesive on the subfloor to help attach it to the joists. While two layers of ply as you indicate are fine, it is overkill for a ceramic tile installation. The extra insurance does not make the floor stiffer along the joists, but does decrease deflection between the joists. A single 3/4" sheet, properly installed exceeds the industry standards for ceramic. A decoupling layer above the heating mats would be more efficient use of your money than a second layer of ply after the tearout. My preference is Ditra. It would provide more protection than the already adequate new 3/4" layer with a 1/2" layer on top. The addition of the heat makes me lean towards the decoupling layer of the Ditra...you don't need the additional stiffness after you put down new 3/4".

Here is something new. I just remembered there is a false wall down below this that comes out 3' on the joists. It is not a great support but does add some support. I forgot about this as I don't think it is a great support but like I said it does add some. The other issue is I wonder if you guys think my tile job will be big. If you call a 4' strip between the cupboards big then yes. Most of the tile will be about a 4' strip. Other parts will be a 6x4 section and another 6x4 section. It is not a big kitchen at all.

I understand that 1/2 on top is not necessary but there will only be 6 pieces to add so cost is not an issue. Will I be better off or will it not make a single bit of difference?
Thanks so much for all your input. I really appreciate it.
My kitchen is only 8' wide. The rest of the span goes into another room on the other side of a wall.
Could I post a picture to show you my kitchen?

The joist span is determined not by the room size but by what is underneath it holding it up, whether that is the rim joist, a support beam, or a structural load-bearing wall. Not all walls are load bearing. Sometimes it takes some good investigation and knowledge to determine that.

Industry standards assume you have no defects in the materials...5/8" subflooring over strong enough joists (L/360 or higher) on 16" centers is considered adequate. Since achieving perfect install is often not possible, a thicker subfloor gives you more margin. With the 1/2" that's there, adding at least 5/8" on top of it should be fine. If you tear out the existing stuff and put down 3/4", you'd be fine. if you wanted stone tile, you'd need a second layer and stronger (stiffer) joists. For a second opinion, go to www.johnbridge.com.

I am getting the bottom 1/2 layer torn out tomorrow and will be putting in some 2x8 sisters and some 2x6 sisters also. One or two will not take either but will take a 2x4 but most will take the other two. That should shore it up a bunch. The sisters won't run the full span but should run at least 7 or the 11' span.

I ended up taking out the crappy 1/2" plywood and have replace it with some nice 3/4"stuff. It has nine layers and both sides are usable. I told them I wanted the strongest stuff they had and they sold it to me. What next? Radiant, thinset, ditra, thinset, and then tile? Or 1/2" ply and then radiant, thinset, ditra, and then thinset and tile? Anyone?
I also sistered the joists and most of them I got about 70% of the joist sistered. Some were more than 80% and some were less than70%.